Blood thrombus, embolus or clots may occur in a person's vasculature system. Sometimes such clots are harmlessly dissolved in the blood stream. Other times, however, such clots may lodge in a blood vessel, where they can partially or completely occlude the flow of blood, referred to as an ischemic event. If the partially or completely occluded vessel feeds blood to sensitive tissue such as, in the brain, lungs, or heart, serious tissue damage may result. Such ischemic events may also be exacerbated by atherosclerosis, a vascular disease that causes the vessels to become narrowed and/or tortuous. The narrowing and/or increased tortuosity of the blood vessels may, in certain circumstances, lead to the formation of atherosclerotic plaque that can cause further complications.
Known embolectomy devices may be used in a variety of applications to remove blood clots or other foreign objects from blood vessels. Such devices include cylindrical scaffold embolectomy devices, such as those illustrated and described in U.S. Pat. No. 8,529,596 to Grandfield, the contents of which are fully incorporated herein by reference.
FIGS. 1A-B illustrate an exemplary prior art embolectomy device 12 that is manufactured and sold by the Neurovascular Intervention Division of Stryker Corp. (http://www.stryker.com/en-us/products/NeurovascularIntervention/index.htm). FIG. 1A shows the embolectomy device 12 in a two-dimensional plane view, and FIG. 1B shows the device 12 a three-dimensional expanded tubular configuration. The embolectomy device 12 is composed of shape memory, self-expandable and biocompatible materials, such as Nitinol. The embolectomy device 12 is preferably manufactured by laser cutting a tube or a sheet of shape memory material. The embolectomy device 12 is coupled to an elongate flexible wire 40 at a proximal end 20 of the device 12. The wire 40 extends proximally from device 12 and is configured to advance and withdraw the embolectomy device 12 through sheaths, catheters and/or patient's vasculature into a target site in a blood vessel. To enhance visibility of the device 12 (e.g., under fluoroscopy) during advancement and withdrawal within the vasculature, the device 12 may be fully or partially coated with a radiopaque material, such as tungsten, platinum, platinum/iridium, tantalum and gold. Alternatively or in conjunction with the use of a radiopaque coating, radiopaque markers 60 may be disposed at or near the proximal end 20 and/or the distal end 22 of the device 12.
As shown in FIG. 1A, the embolectomy device 12 includes a proximal end portion 14, a main body portion 16 and a distal end portion 18, the main body portion including a plurality of longitudinal undulating elements 24 (e.g., wires, struts) with adjacent undulating elements being out-of-phase with one another and connected in a manner to form a plurality of diagonally disposed cell structures 26 extending between the respective proximal and distal end portions of the device. The cell structures 26 in the main body portion 16 and distal end portion 18 of the embolectomy device 12 extend continuously and circumferentially around a longitudinal axis 30 of the device 12 (FIGS. 1A-B).
In particular, the cell structures 26 in the proximal end portion 14 extend less than circumferentially around the longitudinal axis 30 of the device 12. The dimensional and material characteristics of the cell structures 26 of the main body portion 16 are selected to produce sufficient radial force (e.g., radial force per unit length of between 0.005 N/mm to 0.1 N/mm, preferable between 0.030 N/mm to 0.050 N/mm) and contact interaction to cause the cell structures 26, and/or the elements 24, to engage with an embolic obstruction residing in the vasculature in a manner that permits partial or full removal of the embolic obstruction from the patient. As best seen in FIG. 1B, the embolectomy device 12 comprises a lumen 35 and an axial length L1 of about 32 millimeters with the main body portion 16 length L2 measuring about 20 millimeters. The length L2 of the main body portion 16 is generally between about 2.5 to about 3.5 times greater than the length of the proximal end portion 14. Usually, the length L2 is considered the effective length of the device 12 when radial forces are acting upon the deployed device 12 (FIG. 3C) for engagement of embolic obstructions disposed within the vasculature.
FIG. 2 illustrates the embolectomy device 12 of FIGS. 1A-B disposed in a target site of a tortuous vascular anatomy of a patient for capturing an embolic obstruction or clot 75. In an unexpanded or radially compressed configuration (not shown), such as when the embolectomy device 12 is disposed within a delivery catheter 80, the embolectomy device 12 has an unexpanded outer diameter (UOD) between 0.4 to 0.7 millimeters. In a radially expanded configuration (FIGS. 1B-2), the embolectomy device 12 has an expanded outer diameter (EOD) between 2.5 to 5.0 millimeters.
The embolectomy device 12 produces sufficient radial force and contact interaction to cause the strut elements 24 and/or cell structures 26 to engage, snare, encapsulate, capture, pinch and/or entrap the embolic obstruction 75 disposed within a tortuous vasculature, such as blood vessel 70, allowing removal of the embolic obstruction 75 from the patient. The diameter of the main body portion 16 in a fully expanded configuration is about 4.0 millimeters with the cell pattern, elements 24 dimensions and material being selected to produce a radial force of between 0.040 N/mm to 0.050 N/mm when the diameter of the main body portion is reduced to between 1.0 millimeters to 1.5 millimeters. The cell pattern 26, strut dimensions 24 and material(s) are selected to produce a radial force of between 0.010 N/mm to 0.020 N/mm when the diameter of the main body portion 16 is reduced to 3.0 millimeters. Having a strut thickness to width ratio of greater than one promotes integration of the strut elements 24 into the embolic obstruction 75.
Regardless of the technique used to manufacture the embolectomy device 12, the manner in which the strut elements 24 interconnect determines the device's longitudinal and radial rigidity and flexibility. Radial rigidity is needed to provide the radial force needed to engage the clot or embolic obstruction 75, but radial flexibility is needed to facilitate radial compression of the device 12 for delivery into a target site. Longitudinal rigidity is needed to pull, retrieve or withdraw an engaged clot or embolic obstruction 75 from the blood vessel 70, but longitudinal flexibility is needed to facilitate delivery of the device 12 (e.g., through tortuous vasculature).
Embolectomy device 12 patterns are typically designed to maintain an optimal balance between longitudinal and radial rigidity and flexibility for the device 12. However, after deployment of the device 12 into the blood vessel 70 to radially expand into a predetermined diameter, as shown in FIGS. 3A-B, the device 12 is subjected to tensions and forces when withdrawn, as shown in FIG. 3D (i.e., withdrawal force depicted by arrows 31; compressive force depicted by arrows 36; resistance force depicted by arrows 38). In certain applications, the interaction of said forces 31, 36, 38 on the device 12 tend to create a tapered profile 37 reducing the expanded outer diameter (EOD) and the lumen 35 of the device 12 (FIGS. 3C-D).
The tapered profile 37, formed by the compression of the struts 24 when the device 12 is withdrawn, is created at the proximal end portion 14, and usually extends to the main body portion 16 of the device 12, as shown in FIGS. 3C-D. The withdrawal force 31 is exerted to the device 12 through the wire 40 that is attached to the proximal end 20 of the device 12 (FIGS. 1A-B, 2 and 3A-D), which in turn subjects the device 12 to compressive force 36 causing the tapered profile 37, including a reduction on the effective length L2 of the device 12, depicted as a reduced effective length L3 in FIGS. 3C-D. When the device 12 is subjected to the withdrawal force 31, the contact of the device 12 with the blood vessel wall and/or the embolic obstruction produces the resistance force 38 that contributes to the compression and tapered profile 37 of the device 12. As shown in FIG. 3C, the reduced effective length L3 is about 25% smaller than the effective length L2 (reduced percentage shown as “%<L2”). In certain embodiments, the reduced effective length L3 may be 25% to 50% smaller than the effective length L2 (not shown).
The withdrawal 31, compressive 36, and/or resistance 38 forces or combination thereof that cause compression of the struts 24 when the device 12 is withdrawn, and thereby causing the tapered profile 37, reduced effective length L3 and reduced lumen 35 (shown in FIGS. 3C-D) tend to squeeze out the captured embolic obstruction 75 from the device 12, usually, out of the tapered profile 37 and/or the distal end portion 14 of the device 12 (shown in FIG. 4). The squeezed out embolic obstruction 75 or parts thereof, may disengage from the device 12, migrate into other portions of the vasculature within the body, where they can partially or completely occlude the flow of blood. If the partially or completely occluded vessel feeds blood to sensitive tissue such as, the brain, lungs or heart, for example, serious tissue damage, organ failure or death may result.
Accordingly, it would be desirable to prevent compressed tapered profile and reduced effective length of embolectomy devices that may squeeze out captured embolic obstructions into the vasculature of a patient when the device is subjected to withdrawal forces.